Digital multimedia content (e.g. video and/or audio data) must be transported across communication networks in a reliable and timely manner to the end user. Streaming video services (e.g. Internet Protocol television (IPTV), video conferencing, video-on-demand, etc.) are especially sensitive to delay, jitter, or data loss, which can all negatively impact the quality of the end user's experience. Typically, digital multimedia content is encoded prior to transmission across the network. Part 1 (Systems) of the Moving Pictures Expert Group (MPEG-2) standard defines a Transport Stream (TS) for encoding and transporting multimedia content across somewhat unreliable mediums such as broadcast channels, internet protocol networks, etc. The Transport Stream consists of packets that carry video or audio data in their payload. The TS packets are received and decoded to reconstruct the original multimedia content. The Transport Stream protocol is also specified in International Standard ISO/IEC13818-1.
Determining the performance of a network that carries digital multimedia content is an important element to the successful design and operation of such a network. One measure of a network's performance is known as the Media Delivery Index (MDI). The MDI has two components: the delay factor (DF) and the media loss rate (MLR). The MDI is expressed as two numbers separated by a colon: DF:MLR.
The DF component of the MDI is the maximum difference, observed at the end of each network packet, between the arrival of media data and the drain of media data. DF is indicative of the packet jitter. A high value for DF indicates that a larger buffer depth is required to minimize the effects of packet jitter. To calculate DF, consider a virtual buffer VB used to buffer received packets of a stream. Each time a packet Pi, where i>0, arrives during a measurement interval, compute two VB values, VB(i,pre) and VB(i,post) as follows:
                              VB          ⁡                      (                          i              ,              pre                        )                          =                                            ∑                              j                =                1                                            i                -                1                                      ⁢                                                  ⁢                          S              j                                -                      MR            ·                          T              i                                                          (        1        )                                          VB          ⁡                      (                          i              ,              post                        )                          =                              VB            ⁡                          (                              i                ,                pre                            )                                +                      S            i                                              (        2        )            where Sj is the media payload size of the jth packet in the measurement interval, Ti is the arrival time relative to the last received packet in the previous measurement interval, or in the case of the first measurement interval, relative to the first packet. MR is the nominal media rate in bytes per second. VB(i,pre) is the virtual buffer size just before the arrival of Pi, and VB(i,post) is the virtual buffer size just after the arrival of Pi. This calculation is subject to the initial condition of VB(0,post)=VB(0,pre)=0 and VB(1,pre)=−MR·T1 at the beginning of each measurement interval. A measurement interval is defined from just after the time of arrival of the last packet during a nominal period to the time just after the arrival of the last packet of the next nominal period. The length of the measurement interval is arbitrary and may vary from application to application, but is typically selected to be 1 second.
The nominal media rate can be variable or constant. If there is a constant bit rate (CBR), the transmission of the media stream is defined to be constant at the bit level. If there is a variable bit rate (VBR), the rate can vary, but is defined to be piece-wise constant, as defined in the Detailed Description. The prior art DF is well defined for the CBR case only.
The DF is calculated once every measurement interval as follows by finding the difference between the maximum and minimum values of VB:
                    DF        =                                                            max                                  i                  ≥                  0                                            ⁢                              (                                  VB                  ⁡                                      (                                          i                      ,                      post                                        )                                                  )                                      -                                          min                                  i                  ≥                  0                                            ⁢                              (                                  VB                  ⁡                                      (                                          i                      ,                      pre                                        )                                                  )                                              MR                                    (        3        )            Note that the maximum and minimum include the zeros introduced by the initial conditions at i=0.
The MLR is defined as the number of lost or out-of-order packets per second.
For more information regarding the MDI, DF, and MLR measurements, please refer to the following publications: “A Proposed Media Delivery Index (MDI)”, by J. Welch and J. Clark, published in April 2006 by the Internet Engineering Task Force as IETF RFC 4445 and available at the following URL: http://www.rfc-editor.org/rfc/rfc4445.txt; and “IPTV QoE: Understanding and Interpreting MDI Values”, a white paper published by Agilent Technologies on Aug. 30, 2006 and available at the following URL: http://cp.literature.agilent.com/litweb/pdf/5989-5088EN. pdf
An obstacle in calculating the DF is determining the media rate MR. One value to use is the embedded media rate (also referred to as the “elementary stream rate”), which is encoded at a lower level in the program elementary stream (PES) from which the TS packets are created by an MPEG-2 encoder. The embedded media rate is the number of bits per unit time needed to represent the video or audio data only. However, significant processing is required to extract the embedded media rate from these lower level encodings—the received TS packets must first be decoded by the decoder to reconstruct the PES, before the media rate can be extracted from the PES. This process is complex and requires significant hardware and software resources. Furthermore, to perform it in a scalable fashion, the test equipment must parse the PES stream over hundreds of simultaneous video channels, which is typical in an IPTV network. In addition, the embedded media rate is not an accurate representation of the true media rate, because it does not include the necessary MPEG transport overhead required to transmit the payloads.
Therefore, there remains a need for an improved method and apparatus for determining DF.